1. Field of the Invention
This invention relates generally to devices which generate ozone by applying a high voltage across a gap to create a corona discharge, and more specifically to a new type of flat plate ozone generating cell.
2. Description of the Prior Art
Ozone is a very powerful gaseous reactant, and its usefulness has been well established for many years in a wide range of industrial applications. Recently its value in all types of water purification applications has been coming to the fore because of its ability to act as a powerful oxidant, microflocculant and disinfectant without producing toxic side-products.
The most widely used method of generating ozone is to flow dry air or oxygen through a narrow gap bordered on one side by a conductive electrode and on the other side by a dielectric electrode (surfaced on the side which faces away from the gap with an electrical conductor). An alternating high voltage is connected across the electrodes, producing a high voltage field across the gap which creates a corona discharge. This discharge, which is also known as a "silent discharge" or "cold plasma discharge" and is actually composed of many transient microdischarges, converts a percentage of the gas to ozone. The dielectric is necessary to prevent these microdischarges from becoming arcs between the conductive electrodes, which would rapidly destroy the electrode surfaces.
The majority of high quality prior art corona ozone generators have been designed for large-scale industrial-type applications. Today there is a great need in numerous water treatment applications for small stand-alone cells which are very reliable and yet reasonable in cost and easily maintained. Much of the prior art that has addressed this need consists of scaled down versions of previous designs, and because they still retain many of the large-scale design features, are often extremely expensive, and are difficult to assemble and service. Corona ozone generators usually fall into one of two general categories: either the concentric tubular type, in which an elongate annular corona gap is created between a metal tube and a dielectric tube, or the flat plate type, in which a flat corona gap is formed between a metal plate and a dielectric plate. Both types are well known in prior art, with numerous patents having been issued for designs in both categories.
The present invention is an improved flat plate type, and has been created to fulfill the need for a reliable and inexpensive small ozone generator cell which solves the five basic problems inherent in most prior art flat plate designs, which are as follows:
1. Flat plate cells have a rather long perimeter which requires sealing along its entire length. Most cells must utilize seals or gaskets of an elastomeric material which is not entirely resistant to ozone. Ideally, the seal would be of a material which is completely inert to ozone, nitric oxides, and nitric acid. (A by-product of ozone generation is a very small amount of nitric oxides, and even the tiny amount of humidity in the "dry" feed gas usually used converts some of this to nitric acid.) A completely inert elastomeric material is prohibitively expensive, and therefore silicone is often used, which is resistant but not inert to nitric acid and to concentrated ozone, and has to be replaced after about every two years of operation.
2. Seal clamping and plate spacing means usually put pressure on and occasionally break the dielectric plate.
3. Parasitic arcing can occur at the cell perimeter inside the cell. Such arcing erodes the conductive electrode surfaces (and the rougher the surfaces become, the more easily arcing occurs), fatigues the dielectric which hastens its failure, and rapidly destroys the perimeter seal. The arcing occurs most easily and at a lower voltage at the higher ozone and/or corona ion concentrations. In addition, significant amounts of arcing can draw excessive currents from the electronics and high voltage transformer, thereby shortening their life. Arcing also produces large amounts of electromagnetic interference, which can disturb the operation of the power supply and other equipment.
4. Arcing outside of the cell. This external arcing becomes more likely as the ambient humidity increases. It produces ozone outside of the cell, which is extremely undesirable because this ozone ends up inside the electronics cabinet and in the ambient air. Prior art has immersed the cell in oil to prevent such arcing.
5. Dead spaces where gas flow in the corona gap stagnates or temporarily reverses, causing hot spots, reduced ozone generation efficiency, and dielectric fatigue. This problem is inherent when trying to obtain linear flow between the gas entrance and exit holes of most prior art flat plates. The problem can be lessened but not solved through the use of flow baffles. Such baffles usually have to be made of elastomeric materials to be effective, but any elastomer would be quickly degraded in this location because it would be exposed directly to corona bombardment and high ozone concentrations.